Aircraft mounted Ku-band communication antenna systems presently operate in receive only mode. There is a need for an aircraft mounted, Ku-band communication antenna system which can simultaneously transmit and receive from a single aperture. For this system, International Telecommunication Union (ITU) regulatory levels apply such that transmit Effective Isotropic Radiated Power (EIRP) antenna pattern levels cannot exceed ITU regulatory levels for Ku-band satellite interference.
A drawback of the currently used receive-only antennas is that their wide beam widths and high sidelobes cannot meet the beam width and sidelobe requirements for transmit operation under the ITU Ku-band satellite regulations. Use of conventional rectangular slotted waveguide and microstrip-patch array technology cannot be employed because of the high transmit to receive isolation, high efficiency and high cross polarization performance required over the combined transmit and receive operating frequency bandwidth, i.e., about 14.0 GHz to about 14.5 GHz and about 11.2 GHz to about 12.7 GHz respectively.
A large, circular reflector antenna, i.e., approximately 0.9 meters (m) (36 inches) diameter, could be used for the application. Several drawbacks exist, however, for an antenna of this size. The communication antenna(s) is required to be mounted on the external surface of the aircraft fuselage. The vertical height of a 0.9 m diameter antenna creates an aerodynamic vertical drag problem for the aircraft. A further drawback is that aircraft antennas are normally enclosed within a radome in order to protect the antennas and to control aerodynamic drag induced by the antenna(s). As the diameter of an antenna increases, the necessary height and length of the radome increases. The necessary sized radome for a 0.9 m (36 inch) diameter surface mounted reflector antenna produces unacceptable levels of aerodynamic drag.
In addition to the above drawbacks, the effective isotropically radiated power (EIRP) for a single, large antenna and single transmitter is less efficient than an array of smaller antennas and smaller transmitters. Exemplary vertical and horizontal solid state power amplifiers (SSPAs) for a single large antenna producing 20 watts have an efficiency of about 15 percent. The vertical and horizontal SSPAs of four smaller antennas producing an exemplary 5 watts each (for the same total of 20 watts output) have an efficiency of about 25 percent. It is therefore an efficiency drawback to use a single larger antenna if an appropriate number of smaller, more efficient antennas can be employed. Reducing the antenna diameter, however, necessarily reduces the antenna aperture area. To maintain the total aperture area of a 0.9 m diameter reflector antenna by using a greater number of smaller diameter antennas requires balancing several factors. As noted above, using a plurality of smaller diameter reflector antennas decreases drag while increasing efficiency, but also increases system complexity (wiring, receiver differentiation, etc.). The use of a plurality of smaller reflector antennas requires a common support structure, increasing complexity with each antenna to account for the structure and mechanisms required to jointly mount and rotate the assembly. The antennas must be grouped to permit mechanical scanning with the least number of mechanical components, i.e., motors, wiring or gears, to control complexity and weight. A need therefore exists for a wide-band, low drag, mechanically scanned Ku-band communications antenna system which can simultaneously transmit and receive from a single aperture.